1. Field of the Invention
Biological fouling on surfaces is a serious economic problem in many commercial and industrial aqueous process and water handling systems. The fouling is caused by a biomass which is the buildup of microorganisms and/or extracellular substances and by dirt or debris that become trapped in the biomass. Bacteria, fungi, yeasts, diatoms and protozoa are only some of the organisms which cause buildup of a biomass. If not controlled, the biofouling caused by these organisms can interfere with process operations, lower the efficiency of processes, waste energy and reduce product quality.
Cooling water systems used in power-generating plants, refineries, chemical plants, air conditioning systems and other commercial and industrial operations frequently encounter biofilm problems. Biofilm is the buildup of layers of organisms. Cooling water systems are commonly contaminated with airborne organisms entrained by air/water contact in cooling towers as well as waterborne organisms from the systems makeup water supply. The water in such systems is generally an excellent growth medium for these organisms. If not controlled, the biofilm biofouling resulting from such growth can plug towers, block pipelines and coat heat transfer surfaces with layers of slime, and thereby prevent proper operation and reduce equipment efficiency.
Industrial processes subject to problems with biofouling include those used for the manufacture of pulp, paper, paperboard and textiles, particularly water laid nonwoven fabrics. For example, paper machines handle very large volumes of water in recirculating systems called "white water systems". The white water contains pulp dispersion. The furnish to a paper machine typically contains only about 0.5% of fibrous and non fibrous paper making solids, which means that for each ton of paper, almost 200 tons of water pass through the paper machine, most of it being recirculated in the white water system.
These water systems provide an excellent growth medium for microorganisms, which can result in the formation of microbial slime in headboxes, water lines and papermaking equipment. Such slime masses not only can interfere with water and stock flows, but, when they break loose, they can cause spots or holes in the paper as well as web breaks that cause costly disruptions in paper machine operations.
2.Discussion of the Related Art
The control of microbial activity has traditionally been the province of toxic chemicals. Toxic chemical control techniques are well represented in the prior art. U.S. Pat. Nos. 3,959,328, 4,054,542 and 4,285,765 are illustrative of the methods that rely on killing the offending microorganisms with toxic chemicals. Such methods have received the majority of the research effort reported in the prior art because of the logic of eliminating the problem by eliminating the offending organism and because of the large number of organic and inorganic chemicals that will kill microorganisms.
There are certain drawbacks to the use of toxic chemicals. Most of these chemicals that are toxic to microorganisms are also toxic to higher life forms, up to and including humans. The negative effects of these chemicals on the earth's environment and on the food chain is well-documented. Thus any method for controlling microorganisms with toxic chemicals will have some impact on the rest of the population of higher life forms.
The effect of toxic chemicals is, moreover, limited by the organism's own natural defense mechanisms. Planktonic or free-floating organisms are readily destroyed by most chemical agents used to control microorganisms. But sessile, or fixed organisms located on system surfaces, are protected by a polysaccharide covering, and will have some success in warding off the effect of environmental toxins. Thus an increased dose of toxin may well be needed to overcome protection provided by the polysaccharide covering.
Several attempts to control the negative effects of biological activity either avoid the use of toxic chemicals or mitigate their use or impact on the environment. For instance, U.S. Pat. Nos. 3,773,623 and 3,824,184, both to Hatcher et al., disclose the use of the enzyme levan hydrolase to control the formation of bacterial slime in industrial water systems.
U.S. Pat. Nos. 4,055,467 to Christensen, discloses a method for preventing slimes from being deposited on solid surfaces in contact with industrial process waters by using the commercial product, Rhozyme HP 150, which is the enzyme pentosanasehexosanase. This method prevents the buildup of planktonic organisms onto the sessile layers of organisms which are then able to secrete a polysaccharide outer layer. Rhozyme HP-150 is, however, not designed to attack the already accumulated layers of slime that are protected by the polysaccharide cover. These polysaccharides reduce the rate of penetration of the enzyme into the mass of bacteria.
A combination of enzyme and surface active agent has been used in the field of fabric cleaning and stain removal for many years. U.S. Pat. Nos. 3,519,379 to Blomeyer et al. discloses a soaking and laundering process in which a proteolytic enzyme and a peroxy compound are employed along with an organic detergent and an alkaline builder to achieve superior stain removal. U.S. Pat. Nos. 3,985,686 to Barrat, discloses an enzymatic detergent composition containing, inter alia, cationic and anionic surface-active agents and enzymes, particularly proteases.
Blomeyer et al. and Barrat deal with the removal of hydrophobic soil and stains. While the soil and stains are organic, they are not directly biological in their origin and are not in the industrial water system environment. Soil and stain technology relies to some degree on the use of an oxidizing compound to remove the organic material. Oxidizing compounds that have been used to achieve proper control in industrial water systems include chlorine or a chlorine derivative or substitute.
There are references in technical literature to the use of the combination of enzymes and surface active materials to control the growth of biofilm on reverse osmosis membranes used for purification of water (Argo, David G. et al., Aqua Sci. Tech. Rev., "Biological Fouling of Reverse Osmosis Membranes" Vol 6, 1982, pp. 481-491; Whittaker, C. et al., Applied and Environmental Microbiology, "Evaluation of Cleaning Strategies for Removal of Biofilms from Reverse-Osmosis Membranes" Vol 48(2), August 1984, pp. 395-403). The membranes are porous structures made of cellulose acetate.
The single species of microorganism, however, found by Argo et al. and, in the later study identified as greater than 95% Mycobacterium by Whittaker et al., is different from the species that are most prevalent on substrates in industrial systems. The biological reason for this difference is that the cellulose acetate substrate in a spiral wound reverse osmosis membrane is more hospitable to Mycobacterium than to the general class of bacterial organisms found in industrial systems.
The environment studied in the prior art, therefore, was much different than the environment found in most industrial systems that would produce a mixed microflora. In fact, European Patent application number WO 90/02794 by Novo-Nordisk discloses that an enzyme that is required for a specific polysaccharide will be specific to that organism.
Whittaker et al., moreover, noted a marked decrease in the effectiveness of their treatment programs with the increasing age of the microfloral population. This implies that the organisms were able to resist the treatment more effectively as their numbers increased.
Argo et al. disclose that systems treated with chlorine were more effectively treated with the enzyme program. Whittaker et al. observed that the use of a commercial cleaning product containing certain enzymes, surface active compounds and bleach was the most effective in controlling biofilm of those tested. Whittaker et al. also notes, however, that no treatment or combination of treatments was completely effective or effective at all stages of biofilm development. Argo additionally points out that no cleaner tested consistently removed all the biofilm from all the membranes.
U.S. Pat. Nos. 4,936,994 to Wiatr discloses a method of removing slime from slime-covered surfaces of cooling towers with an enzyme preparation which combines the activities of cellulase, alphaamylase and protease. Wiatr requires the combination of the three enzymes and sets forth at column 3, lines 41-44, that none of the enzymes alone would remove enough slime to be effective.
U.S. Pat. 4,684,469 to Pedersen et al. and U.S. Pat. No. 4,370,199 to Orndorff both disclose methods for controlling biofilm with an enzyme-catalyzed biocide. These methods do not eliminate the use of a toxicant, but merely use the enzyme component to improve the performance of a toxic chemical.